Next Generation Nuclear Plant Phenomena Identification and Ranking Tables (PIRTs) (NUREG/CR-6944) - Volume 4: High-Temperature Materials PIRTs
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Publication Information
Manuscript Completed: October 2007
Date Published: March 2008
Prepared by:
W.R. Corwin – Panel Chair
Panel Members:
R. Ballinger (MIT)
S. Majumdar (ANL)
K.D. Weaver (INL)
Oak Ridge National Laboratory
P.O. Box 2008
Oak Ridge, TN 37831-6170
S. Basu, NRC Project Manager
NRC Job Code N6376Office of Nuclear Regulatory Research
Abstract
The Phenomena Identification and Ranking Table (PIRT) technique was used to identify safety-relevant/safety-significant phenomena and assess the importance and related knowledge base of hightemperature
structural materials issues for the Next Generation Nuclear Plant (NGNP), a very high
temperature gas-cooled reactor (VHTR). The major aspects of materials degradation phenomena that may
give rise to regulatory safety concern for the NGNP were evaluated for major structural components and
the materials comprising them, including metallic and nonmetallic materials for control rods, other reactor
internals, and primary circuit components; metallic alloys for very high-temperature service for heat
exchangers and turbomachinery, metallic alloys for high-temperature service for the reactor pressure
vessel (RPV), other pressure vessels and components in the primary and secondary circuits; and metallic
alloys for secondary heat transfer circuits and the balance of plant. These materials phenomena were
primarily evaluated with regard to their potential for contributing to fission product release at the site
boundary under a variety of event scenarios covering normal operation, anticipated transients, and
accidents. Of all the high-temperature metallic components, the one most likely to be heavily challenged
in the NGNP will be the intermediate heat exchanger (IHX). Its thin, internal sections must be able to
withstand the stresses associated with thermal loading and pressure drops between the primary and
secondary loops under the environments and temperatures of interest. Several important materials-related
phenomena related to the IHX were identified, including crack initiation and propagation; the lack of
experience of primary boundary design methodology limitations for new IHX structures; and
manufacturing phenomena for new designs. Specific issues were also identified for RPVs that will likely be too large for shop fabrication and transportation. Validated procedures for on-site welding, postweld
heat treatment (PWHT), and inspections will be required for the materials of construction. High-importance
phenomena related to the RPV include crack initiation and subcritical crack growth; field
fabrication process control; property control in heavy sections; and he maintenance of high emissivity of
the RPV materials over their service lifetime to enable passive heat rejection from the reactor core. All
identified phenomena related to the materials of construction for the IHX, RPV, and other components
were evaluated and ranked for their potential impact on reactor safety.
